The invention relates to an illumination system of a microlithographic projection exposure apparatus comprising (a) a plurality of channels, each channel guiding a partial beam and at least one channel comprising at least one defect, and (b) at least one optical element arranged within the at least one channel having the at least one defect, the optical element being adapted to at least partially compensate the at least one defect of the partial beam of the channel.
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1. An illumination system configured so that, during use of the illumination system, the illumination system guides a plurality of partial beams of light, each partial beam of light having a corresponding channel, the illumination system comprising:
an optical element,
wherein:
the optical element comprises local persistent modifications in an area of the optical element outside an optically relevant area of the optical element;
the local persistent modifications in the optical element produce a strain-induced birefringence in the optical element;
the optical element is configured so that, during use of the illumination system;
a) the partial beam of light in a first channel impinges on the optical element; and
b) the strain-induced birefringence of the optical element at least partially compensates a variation of a polarization of the partial beam of light of the first channel; and
the illumination system is a microlithographic illumination system.
2. The illumination system of
3. The illumination system of
4. The illumination system of
5. The illumination system of
6. The illumination system of
7. The illumination system of
the illumination system comprises a two dimensional mirror array comprising a plurality of mirrors; and
each mirror of the plurality of mirrors of the two dimensional mirror array is tiltable about two axes so that, during use of the illumination system, the partial beams of light of the first channel is superimposed with a partial beam of light of second channel in a single spot so that a retardation of the superimposed partial beams of light compensates retardation of the illumination system.
8. The illumination system of
9. The illumination system of
10. The illumination system of
11. The illumination system of
12. The illumination system of
13. The illumination system of
14. The illumination system of
15. The illumination system of
16. The illumination system of
17. The illumination system of
18. The illumination system of
are arranged in a one-dimensional row or are arranged in a two-dimensional rectangular matrix;
have a diameter which is adjustable to a diameter of the partial beams of the first and second channels; and
have a distance which is adjustable to a distance between the partial beam of light each of the first and second channels.
19. The illumination system of
20. The illumination system of
21. The illumination system of
22. The illumination system of
23. The illumination system of
24. An apparatus, comprising:
an illumination system according to
a projection objective,
wherein the apparatus is a microlithographic projection exposure apparatus.
25. A method comprising:
using an illumination system of
using a projection objective to image at least a portion of the mask onto a photosensitive material.
26. The method of
27. The method of
28. The illumination system of
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The present application is a continuation of, and claims benefit under 35 USC 120 to, international application PCT/EP2012/055621, filed Mar. 29, 2012. The entire disclosure of this application is incorporated by reference herein.
The present invention relates to the field of compensating a defect of a channel of a microlithographic projection exposure system.
Microlithographic projection exposure systems are used for the production of microstructured components, in particular semiconducting components such as integrated circuits (ICs). A microlithographic projection exposure system comprises as essential components a light source, an illumination device or illumination system and a projection objective or a projection system. In modern projection exposure systems which use electromagnetic radiation of the deep ultraviolet (DUV) wavelength range, the light source is typically an excimer laser system (a krypton fluoride (KrF) excimer laser for the 248 nm, an argon fluoride (ArF) laser for the 193 nm, or a fluoride (F2) excimer laser for the 157 nm wavelength).
The properties of the illumination system determine the imaging quality and the wafer throughput which can be achieved with the microlithographic projection exposure system. The illumination system has to be capable of forming a light beam from the light source for various possible illumination modes or settings. Various settings as for example an annular field illumination and/or dipole or quadrupole off-axis illuminations having different degrees of coherence are used for generating an optimal imaging contrast of the structure elements of the photolithographic mask in the photosensitive layer arranged on a substrate. At the same time, the projection exposure system has to have a reasonable process window. For example, off-axis oblique illuminations can be used in order to increase the depth of focus (DoF) via a two-beam interference and also to increase the resolving power of the overall system.
The illumination system has to generate the various settings with the highest efficiency as the effort and cost drastically increase for the generation of electromagnetic radiation with decreasing wavelength, in particular in the DUV wavelength range. Furthermore, it is mandatory that the optical intensity distribution is as uniform as possible across the illumination mode since any inhomogenity reduces the critical dimension (CD) of the feature elements which are to be imaged on a substrate.
In order to fulfil these requirements, the optical beam of the light source is separated or split into a number of partial beams which are individually shaped and/or directed in various channels by micro-structured optical components within the optical illumination system. Microlithographic illumination systems which use different principles for splitting and guiding the partial beams are for example disclosed in the US 2004/0 108 167 A1 and the WO 2005/026 843 A2.
The term “channel” means here and in the following a volume within the illumination system through which a partial beam travels from the location where it is generated by splitting of the input beam to the location where it is superimposed or combined with other partial beams.
The projection objective of the microlithographic projection exposure system collects the light transmitted through the mask and focuses it onto a photosensitive layer or photoresist dispensed on a substrate which is arranged in the focus plane of the projection objective. The substrate is often a semiconducing wafer, as for example a silicon wafer.
As a result of the constantly increasing integration density in the semiconductor industry, photolithographic projection exposure systems have to project smaller and smaller structures onto the photoresist. In order to fulfil this demand, as already mentioned, the exposure wavelength of projection exposure systems has been shifted from the near ultraviolet across the mean ultraviolet into the deep ultraviolet region of the electromagnetic spectrum. Presently, a wavelength of 193 nm is typically used for the exposure of the photoresist on wafers. As a consequence, the manufacturing of microlithographic projection exposure systems with increasing resolution is becoming more and more complex, and thus more and more expensive as well. In the future, projection exposure systems will use significantly smaller wavelengths in the extreme ultraviolet (EUV) wavelength range of the electromagnetic spectrum (e.g. in the range of 10 nm-15 nm).
At a given wavelength the resolution of a projection exposure system can be augmented by increasing the numerical aperture (NA) of its projection system. M. Totzeck et al. discuss in the article “Polarization influence on imaging”, J. Microlith., Microlab., Microsyst., 4(3) (July-September 2005), p. 031108-1-031108-15) that for high NA projection systems the polarization of the illumination beam has a significant influence on the resolution of a projection exposure system.
Therefore, in order to be able to control the degree of coherence of the optical beam exiting a microlithographic illumination system, it is necessary to control its polarization state. Various approaches are already known for adjusting a predetermined polarization distribution in the pupil plane and/or in the mask plane of the illumination system as well as in the projection system in order to optimize the image contrast. Some not exhaustive examples are listed in the following: WO 2005/069081 A2, WO 2005/031467 A2, U.S. Pat. No. 6,191,880 B1, US 2007/0146676 A1, WO 2009/034109 A2, WO 2008/019936 A2, WO 2009/100862 A1, EP 1 879 071 A2, and DE 10 2004 011 733 A1
The documents mentioned above describe a control of the polarization of the overall beam or of some sub-beams comprising many partial beams of individual channels of the illuminations system. On the other hand, there may be defects as for example polarization defects of individual partial beams caused by defective or weak optical components within individual channels. The superposition of partial beams where one or several partial beams may have a changed or even an indefinite polarization state can lead to an unpredictable polarization state of the overall beam. This situation results in a reduction of the intensity in preferred state (IPS) which may fall below a predetermined threshold.
It is therefore one object of the present invention to provide an apparatus and a method for compensating defects of partial beams within the channel of the partial beams.
According to a first aspect of the invention, an illumination system of a microlithographic projection exposure apparatus comprises (a) a plurality of channels, each channel guiding a partial beam and at least one channel comprising at least one defect, and (b) at least one optical element arranged within the at least one channel having the at least one defect, the optical element being adapted to at least partially compensate the at least one defect of the partial beam of the channel.
By compensating a defect within a channel of a partial beam, it is avoided that the local defect of a partial beam may deteriorate the quality of the beam formed by a superposition of several of the partials beams exiting the illumination system to illuminate the mask. In particular, the superposition of several well behaved partial beams and a single defective partial beam may result in an impairment of a second quantity of the overall optical beam exiting the illumination system. Such an impairment may lead to complex correction measures for the illumination beam(s) of an illumination system.
Since the optical element only minimally affects the beam characteristics of the partial beam in the respective channel, the insertion of the optical element does not require compensative measures for the partial beam. On the other hand, the compensation of the defect effectively increases the intensity in preferred state (IPS) of the beam(s) exiting the illumination system.
In a further aspect, the at least one defect comprises a variation of the polarization of the partial beam in the at least one channel. In another aspect, the at least one optical element at least partially compensates the at least one defect by modifying a polarization of the partial beam.
One example of a defect in a channel of an illumination system is a variation of the polarization of the partial beam. Sources for such a polarization variation may be reflection and/or transmission at interfaces and coatings and/or a birefringence of optical components in the channel of the optical beam. Birefringence may be intrinsic or material birefringence and/or may be induced by a strain caused by the mounting of the optical component. By selectively modifying the polarization of a partial beam, the variation of the polarization can at least partially be compensated.
According to a further aspect, the at least one optical element comprises a strain induced birefringence which at least partially compensates the variation of the polarization of the partial beam. In another beneficial effect, the strain induced birefringence comprises introducing at least one arrangement of local persistent modifications in an area of the optical element outside of its optically relevant area.
It is well known that the introduction of a local deformation in a transparent material causes a local strain induced birefringence variation. Based on a model which describes an induced local deformation as a function the parameters of the laser beam used for the introduction of the local deformation, the strain distribution which is locally induced in a material of an optical element can be controlled. Therefore, a predetermined local strain distribution can be introduced in the optical element. This strain distribution results in a respective birefringence distribution.
In a preferred embodiment, one or several arrangements of local persistent modifications are introduced in an area of the optical element which is optically not relevant. The far reaching strain components extend in the optically relevant area of the optical element and cause a strain induced birefringence distribution which at least partially compensates the optical defect in the respective channel. The arrangement of local persistent modifications can be calculated from the required birefringence distribution with the aid of a computer system.
The configuration in which the arrangement(s) of local persistent modifications are introduced in an area outside of the optically relevant area of the optical element has the advantage that the arrangement of local persistent modifications cannot have an impact on photons of the partial beam. Moreover, the arrangement of local persistent modifications cannot degrade the optical properties of the optically relevant area of the optical element.
The term “essentially” means in this description a variation of a quantity which is below the resolution limit of state of the art measurement technique.
The optically relevant area of an optical element is the area through with a beam or a partial beam traverses the optical element or from which a beam or a partial beam is reflected. The optically not relevant area is given by all other area portions of the optical element.
In a further aspect, a mirror of a two-dimensional mirror array directs a partial beam passing through the at least one optical element in an outer edge of a pupil in agreement with a predetermined target pupil.
The retardation caused by the birefringence shows a symmetrical variation across the beam within the pupil plane. It increases from the pupil centre toward the pupil edge. Thus, by directing the partial beam or the partial beams traversing the at least one optical element and thus having acquired a defined retardation toward the pupil edge, the retardation in the pupil edge can be compensated. This process has to take the constraint into account that the intensity distribution in the pupil plane has to fulfil a predetermined target distribution. However, it is not necessary that the defect in the pupil plane comprises a symmetric variation across the pupil in order to be compensated in the defined illumination system.
According to a further beneficial aspect, the strain induced birefringence of the at least one optical element comprises a fast axis having a fixed direction.
Introducing a strain induced birefringence in the optical element having a fast axis with a fixed direction allows a space-resolved correction of the polarization variation.
In another aspect, the strain induced birefringence of the at least one optical element comprises a retardation in the optical relevant area of the at least one optical element of 1 nm-10 nm, preferably 2 nm-8 nm, and most preferably 3 nm-6 nm.
In another preferred aspect, the at least one optical element maximizes a modification of the polarization of the partial beam by selecting an orientation of the fast axis of the at least one optical element with respect to a polarisation of the partial beam in the channel.
The described optimization process is easily performed if the partial beam has a linear polarization and the optical element has a fast axis with a fixed direction. However, the defect compensation can also be performed if the partial beam has an arbitrary polarization and the strain induced birefringence does not generate a fixed fast axis.
In another beneficial aspect, the illumination system is adapted to superimpose partial beams of at least one first channel and at least one second channel in a single spot, so that the retardation of the superimposed partial beams compensates the retardation of the illumination system.
This feature is advantageous as it allows compensating defects of several channels whose partial beams superimpose in a single spot to form an illumination beam exiting the illumination system by inserting a specifically designed optical element in one of the channels of the partial superimposed beams. Consequently, the defect compensation of a multitude of channels where some or all of the channels have a defect is simplified to a large extent.
In a further aspect, the polarization of the partial beam of the at least one first channel is rotated by a predetermined amount with respect to the polarization of the partial beam of the at least one second channel.
According to a further aspect, the compensation of the at least one defect increases an intensity in preferred state of a beam exiting the illumination system.
The intensity in preferred state (IPS) is an important characteristic for the imaging quality of a projection exposure system. The IPS is therefore defined by a threshold value (e.g. IPS >97%) which restricts the loss of usable photons to a very low percentage.
In another aspect, the at least one optical element is adapted to comprise an actuator to induce strain in the optically relevant area of the at least one optical element. In a further aspect, the actuator comprises a piezo-electric element.
By combining the optical element with an actuator, a temporarily changing strain can be applied to the optical element which allows a rapid adaptation of the birefringence to a new situation. The actuator may change the retardation leaving the fast axis untouched, or it may be arranged to modify both, the orientation of the fast axis and the retardation. It is also conceivable to arrange two or more actuators to the optical element.
According to another aspect, the at least one optical element is adapted to be dynamically inserted in the at least one channel during an operation of the illumination system.
This feature enables an optimization of the IPS of the projection exposure system during its operation. It requests that the optical element does neither essentially vary the optical intensity of the partial beam nor change its phase relationship.
In still a further aspect, the at least one optical element comprises a multitude of optical elements being fabricated with different fixed fast axes and/or different amounts of retardations.
For example, there may be defects in a channel of a microlithographic illumination system having various amounts of material birefringence. Therefore, it may be favourable to generate optical elements having various arrangements of local persistent modifications which are adapted to the different defects occurring in the channels of the illumination system.
According to another aspect, the multitude of optical elements is adapted to be inserted in a multitude of channels each one having a defect so that the intensity in preferred state of a beam exiting the illumination system is maximized.
The microlithographic illumination system has a multitude of channels. Therefore, it is possible that more than one channel has a defect. The defect in two or more channels may have essentially the same defect amount or the defects of the different channels may have different defect amounts. These defects can be compensated by inserting optical elements in the respective defective channels which best compensate the respective defect. The defect compensation is controlled by the IPS of the superimposed partial beams.
In another aspect, the at least one optical element is adapted to comprise a polarizer which changes the polarization of the partial beam by a predetermined amount.
By introducing an arrangement of local persistent modifications in an area optically not used by the polarizer, the polarizer can perform a predetermined polarization manipulation at the partial beam and can simultaneously compensate a defect caused by a polarization variation within the channel. Such a configuration may save space in the channels of the microlithographic illumination system.
In a further aspect, the at least one optical element comprises a mirror for reflecting the partial beam. According to yet another aspect, the at least one optical element comprises a lens for deflecting the partial beam.
As already mentioned, a micolithographic illumination system typically comprises many different channels each one guiding a partial beam. Consequently, there is often less space between the various partial beams. It may therefore be beneficial to combine the defect compensation with a further function of an optical element in order to save space in the illumination system.
It is also conceivable to combine the just discussed approach with the insertion of one or more optical elements specifically designed for the defect compensation in a channel. It can be beneficial to add a predetermined amount of defect compensation to optical elements of the specific channels as for example channels close to the edges and/or corners of the channel matrix and insert additional optical elements only to a few channels having a specific defect.
According to another aspect, the at least one optical element is adapted to comprise at least one plate having at least two optically relevant areas for at least two channels, the optically relevant areas are arranged between areas of the at least one plate which are optically not relevant.
As the channel arrangement of a microlithographic illumination system typically comprises many channels, there may be situations in which it is cumbersome to individually compensate defects in a number of channels. This approach of the defect compensation of individual channels may be limited due to space restrictions if a number of adjacent channels of the channel arrangement have a defect. Therefore, it may be useful to generate a plate which can compensate defects of a number of channels.
In still another aspect, the at least two optically relevant areas are arranged in a one-dimensional row or a two-dimensional rectangular matrix, the at least two optically relevant areas have a diameter which is adjusted to a diameter of the partial beams, and the at least two optically relevant areas have a distance which is adjusted to a distance between the partial beams of the at least two different channels.
In a further aspect, the plate comprises at least one optically relevant area being adapted to at least partially compensate the at least one defect and at least one optical relevant area not having a strain induced birefringence.
Similar to an optical element for a single channel, a plate may integrate various functions as for example defect compensation and generating a predetermined polarization variation.
In still another aspect, the plate comprises at least two optically relevant areas being adapted to compensate at least two different defects.
According to a further aspect, the at least two optically relevant areas of the plate comprise a polarizer which change the polarization of the partial beam by a predetermined amount.
In yet another aspect, ultra-short laser pulses are used to introduce the at least one arrangement of local persistent modifications.
In a further beneficial aspect, the plate is adapted to be moved and/or to be rotated essentially perpendicular to the beam directions of the at least two partial beams.
Similar to an optical element for a single channel, this feature enables an optimization of the defect compensation by adjusting the plate to a position within the channel matrix at which the IPS loss is minimized.
According to another aspect, the plate is adapted to be dynamically inserted in a beam path of the partial beams of the least two channels during the operation of the illumination system.
In still a further advantageous aspect, a method for compensating at least one defect in at least one channel of an illumination system of a microlithographic projection exposure system uses an illumination system of any of the above described aspects.
Finally, another aspect further comprises the step of inserting at least one optical element in at least one channel having a defect which is best compensated by the at least one optical element.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
In order to better understand the present invention and to appreciate its practical applications, the following figures are provided and referenced hereafter. It should be noted that the figures are given as examples only and in no way limit the scope of the invention.
In the following, the present invention will be described hereinafter in more detail with reference to the accompanying figures, in which exemplary embodiments of the invention are illustrated. However, the present invention may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and will convey the scope of the invention to persons skilled in the art.
The beam expansion unit 120 increases the diameter of the laser beam 145 from the millimeter to the centimeter range. For example, the beam entering the expansion unit 120 can have a size of 15 nm×20 nm and the beam exiting the expansion unit 120 may have for example dimensions of 80 mm×80 mm.
In the exemplary embodiment of
The projection objective 140 images the light which traversed the mask 150 onto a photosensitive layer 170 arranged on a substrate 160. The substrate 150 is often a silicon wafer.
In an exemplary embodiment, the expanded beam has dimensions of 80 mm×80 mm. The two-dimensional raster array of micro-lenses 215 splits the beam for example in squares of 4 mm×4 mm which results in an array or in a matrix of 20×20 partial beams 205. This means that in the discussed example a channel 220 of a partial beam 235 has dimensions perpendicular to the beam direction of 4 mm×4 mm. In
Downstream the micro-lens array 225 a polarization plate 230 is inserted in some channels 220 of partial beams 235. The polarization plate 230 changes the linear polarization of the partial beams in a defined manner for example by rotating the polarization by a predefined angle and/or by forming circularly polarized partial beams 235. Thus, the polarization plate 230 allows to setting up a predetermined polarization state of the beam(s) 285 exiting the illumination system 130 to illuminate the mask 150.
Each of the partial beams 235 strikes a respective mirror 240 of the two dimensional mirror array 250. In the discussed example, the mirrors 240 of the mirror array 250 are flat and have a size of 3 mm×3 mm. Each of the mirrors 240 can be tilted about two tilt axes which are aligned perpendicular to each other. The tilting movement of each of the mirrors 240 can be individually controlled by a control unit 260 which is connected to the mirror array 250 via the electrical connection 255.
The individual mirrors 240 are located in a square region of 4 mm×4 mm and the square regions are located beside one another on the two-dimensional mirror array 250. The number of the individual mirrors 240 is adjusted to the number of the partial beams 235 and is in the discussed example ≧400. The mirror array 250 is arranged on the deflection mirror 132 of the first part 131 of the illumination system 130. Instead of the mirror array 250 an array of controllable reflective diffraction gratings can also be used to deflect the partial beams 235 in a predetermined direction.
The optical components 265 are used to form or shape the partial beams 235 reflected from the mirrors 240. For this purpose, at least some of the optical components 265 are movable along the optical axis of the illumination system 130 (not shown in
In the discussed example, a raster element 270 is arranged in the pupil-shaping surface or close to it. The raster element 270 has a two-dimensional array of diffractive or refractive optical elements which can fulfil several functions. The raster element 270 shapes the incident partial beams 235. Further, it forms partial beams leaving the raster element 270 which superimpose in a subsequent field plane and thus mixing the partial beams to homogeneously illuminate the exit pupil of the illumination system 130 in which the mask or reticle 150 is arranged (not indicated in
The lens 280 symbolizes the objective arranged in the third part 136 of the illumination system 130 which projects the homogenizing field plane having the mask system onto the reticle plane 290 in which the mask 150 is arranged.
The mirror array 250 changes the angular distribution of the partial beams 235 incident on the mirrors 240 of the mirror array 250, and thus rearranges the optical intensity distribution of the expanded beam 205. In the example schematically illustrated in
Furthermore, the mirror array 250 can switch individual partial beams 235 by a respective tilting of an individual mirror in a respective portion of the pupil plane. This provides the possibility to set a desired or predetermined retardation distribution within the pupil plane of the illumination system 130 to compensate a respective retardation distribution within the pupil plane. The described retardation compensation process has additionally to fulfil the requirement that the optical intensity is homogeneously distributed within the predetermined setting, which is a dipole setting in the example of
On their path from the two-dimensional raster array 215 to the two-dimensional raster array 270, the partial beams 235 traverse several optical elements as for example the micro-lens array 225, the polarization plate 230, and the optical components 265. Moreover, the partial beams 235 are reflected from the mirrors 240. At each of these interactions of the partial beams 235 with the optical components 215, 225, 240, 230, 265, 270, a partial beam 235 may be distorted. For example, photons of the partial beam 235 may be absorbed or scattered from the beam which results in a decrease of the intensity of the partial beam 235. Further, the phase relationship within the partial beam 235 may be distorted by the interaction with the optical components 215, 225, 230, 240, 265, 270. Moreover, the polarization of the partial beam 235 may be distorted by the reflection and/or transmission of the partial beam 235 at the interfaces and/or coatings of the components 215, 225, 230, 240, 265, 270. Additionally, intrinsic or material birefringence of the optical elements 215, 225, 230, 265, 270 can change the polarization state of a partial beam 235.
The following description concentrates on the compensation of polarization variations which the partial beam 235 acquires during its path in the respective channel 220. It is however appreciated that the discussed measures can also be used to compensate other defects of a partial beam 235.
In the next step, the fabrication of an optical element is explained which is able to compensate a polarization variation of a partial beam 235. Then, examples of the insertion of the optical element in the channel 220 of a partial beam 235 are discussed.
The retardation Δ of an optical element 215, 225, 230, 265, 270, 500 having a material birefringence is determined by the thickness d of the optical element 215, 225, 230, 265, 270, 500 and the refractive indices of its fast nF and slow axis nS according to the equation:
Δ=d·(nS−nF)=d·Δn=d·δ (1)
where δ is called birefringence.
The retardation Δ caused by the material birefringence of the optical elements 215, 225, 230, 265, 270 varies the polarization state of the partial beams 235, and thus results in a variation of the polarization of the illumination beams 285 across the illumination field 290 of the mask 150. As a result, a polarization variation exemplarily dispersed in
This high IPS specification threshold results in a retardation budget of the overall projection exposure system 100 including also all external contributions in the range of approximately 10 nm. Thus, the IPS specification sets new birefringence requirements to every optical element 215, 225, 230, 265, 270 of the illumination system 130 as well as for the projection system 140 of the microlithographic projection exposure system 100. Furthermore, it also makes high demands on the polarization variations induced by the mask 150, the photoresist 170 and the pellicle of the mask 150 (not shown in
The apparatus 400 includes a pulse laser source 430 which produces a beam or a light beam 435 of pulses or laser pulses. The laser source 430 generates light pulses or laser pulses of a variable duration. The pulse duration may be as low as 10 fs but may also be continuously increased up to 100 ps. The pulse energy of the light pulses generated by the pulsed laser source 430 can also be adjusted across a huge range reaching from 0.01 μJ per pulse up to 10 mJ per pulse. Further, the repetition rate of the laser pulses comprises the range from 1 Hz to 100 MHz. In a preferred embodiment the light pulses may be generated by a Ti:Sapphire laser operating at a wavelength of 800 nm. However, the methods described in the following are not limited to this laser type, principally all laser types may be used having a photon energy which is smaller than the band gap to the material of the optical element 410 and which are able to generate pulses with durations in the femtosecond range. Therefore, for example Nd-YAG laser or dye laser systems may also be applied.
The apparatus 400 may also comprise more than one pulse laser sources 430 (not shown in
The following table represents an overview of laser beam parameters of a frequency-doubled Nd-YAG laser system which is used in one aspect for introducing an arrangement of local persistent modifications in an optical element 410. A single local persistent modification locally modifies a density of the optical element 410. The locally modified density of the optical element 410 is discontinuously modified within at least one small volume of the optical element 410, wherein the at least one small volume is called a pixel. Furthermore, the locally modified density of a single local persistent modification induces a strain distribution around the local persistent modification of the induced pixel. By introducing or writing many pixels in a dedicated arrangement in the material of the optical element 410 a desired strain distribution can be generated.
It is known that the induced change in the impermeability Δβij linearly depends on the strain induced in the material, wherein the impermeability β and the permittivity ε are linked by:
This dependence can be expressed with the aid of the components of the stress optic matrix:
Therefore, the strain induced in the material of the optical element 215, 225, 230, 265, 270, 500 by introducing or writing pixels in the optical element 215, 225, 230, 265, 270, 500 is directly linked to the retardation Δ of an optical beam or the partial beam 235 in the material and is given by the equation:
where d is again the thickness of the optical element 215, 225, 230, 265, 270, n0 is the refractive index of the isotropic material of the optical element 215, 225, 230, 265, 270, 500 and βij are the components of the impermeability matrix of the material of the optical element 215, 225, 230, 265, 270, 500.
In the following, one set of laser beam parameters characterizing a specific laser pulse is also called a writing mode, as the laser beam is focussed into the optical element 215, 225, 230, 265, 270, 500 and laser pulses are “written” into the material of the optical element 410. Each set of laser beam or laser pulse parameters or each writing mode induces a local deformation in the optical element 215, 225, 230, 265, 270, 500 which is characteristic or specific for this parameter set. In other words, each parameter set for a laser pulse or each writing mode generates its specific deformation in the material of the optical element 410.
The effect of a laser pulse on the optical element 215, 225, 230, 265, 270, 500 is in the following described in form of a parameter called mode signature (MS). In this concept, the area of the optical element 215, 225, 230, 265, 270, 500 is divided into small elementary areas, preferably small rectangles or squares. The mode signature describes the distortion or deformation of an elementary area due to the action of a laser pulse or a sum of laser pulses.
TABLE 1
Numerical values of selected laser beam parameters for a
frequency-doubled Nd-YAG laser system
Overview
Parameter
Numerical value
Unit
Pulse energy
0.05-5
μJ
Pulse length
0.05-100
ps
Repetition rate
1-10 000
kHz
Pulse density
1 000-10 000 000
mm−2
NA
0.1-0.9
Wavelength
532
nm
The following tables indicate parameters for differently influencing the arrangement of local persistent modifications and thus the induced strain distribution in the material of the optical element 410. Table 2 presents parameters of a frequency-doubled Nd-YAG laser system for a mode of introducing or writing pixels called standard process window (Std PW).
TABLE 2
Numerical values of selected laser beam parameters for a Nd-YAG
laser system for a standard process window
Std PW (standard process window)
Parameter
Numerical value
Unit
Pulse energy
0.45
μJ
Pulse length
8
ps
Repetition rate
50
kHz
Pulse density
1 000-100 000
mm−2
NA
0.3
Wavelength
532
nm
Table 3 summarizes parameters of a mode called low registration process window (Low-Reg PW) again of an embodiment using a frequency-doubled Nd-YAG laser system. This operation mode of the laser system 430 uses light pulses having a lower energy than the Std PW, but introduces a higher pixel density.
TABLE 3
Numerical values of selected laser beam parameters for a Nd-YAG
laser system for a low registration process window (LowReg PW)
LowReg PW (low registration process window)
Parameter
Numerical value
Unit
Pulse energy
0.32
μJ
Pulse length
8
ps
Repetition rate
50
kHz
Pulse density
5 000-500 000
mm−2
NA
0.3
Wavelength
532
nm
The steering mirror or steering system 490 directs the pulsed laser beam 435 into the focusing objective 440. The objective 440 focuses the pulsed laser beam 435 into the optical element 410. The NA (numerical aperture) of the applied objective 440 depends on the predetermined spot size of the focal point and the position of the focal point within the material of the optical element 410. As indicated in table 1, the NA of the objective 440 may be up to 0.9 which results in a focal point spot diameter of essentially 1 μm and a maximum intensity of essentially 1020 W/cm2.
The apparatus 400 also includes a controller 480 and a computer system 460 which manage the translations of the two-axis positioning stage of the sample holder 420 in the plane of the x and the y direction. The controller 480 and the computer system 460 also control the translation of the objective 440 perpendicular to the plane of the chuck 420 (z direction) via the one-axis positioning stage 450 to which the objective 440 is fixed. It should be noted that in other embodiments of the apparatus 400 the chuck 420 may be equipped with a three-axis positioning system in order to move the optical element 410 to the target location and the objective 440 may be fixed, or the chuck 420 may be fixed and the objective 440 may be moveable in three dimensions. It should be further noted that manual positioning stages can also be used for the movement of the optical element 410 to the target location of the pulsed laser beam 435 in the x, the y and the z direction and/or the objective 440 may have manual positioning stages for a movement in three dimensions.
The computer system 460 may be a microprocessor, a general purpose processor, a special purpose processor, a CPU (central processing unit), a GPU (graphic processing unit), or the like. It may be arranged in the controller 480, or may be a separate unit such as a PC (personal computer), a workstation, a mainframe, etc. The computer system 460 may further comprise I/O (input/output) units like a keyboard, a touchpad, a mouse, a video/graphic display, a printer, etc. In addition, the computer 460 may also comprise a volatile and/or a non-volatile memory. The computer system 460 may be realized in hardware, software, firmware, or any combination thereof. Moreover, the computer system 460 may control the laser source 430 (not indicated in
Further, the apparatus 400 may also provide a viewing system including a CCD (charge-coupled device) camera 465 which receives light from an illumination source arranged to the chuck 420 via the dichroic mirror 445. The viewing system facilitates navigation of the optical element 410 with respect to the target position. Further, the viewing system may also be used to observe the formation of the pixel arrangement in the optical element 410 by the pulse laser beam 435 of the light source 430.
In the example discussed in the context of
The optically not relevant area 520 is used to introduce one or several arrangements 530, 540 of local persistent modifications. The arrangements 530, 540 are indicated in
This approach has the advantage that the polarization variation of the optical elements 215, 225, 230, 265, 270 of individual channels 220 has not to be analysed. Further, it is not necessary to determine individual pixel arrangements 50, 540 for each of the optical elements 500. Therefore, this approach is discussed in the following examples.
However, it has to be stressed that it is in a second approach also possible to measure the polarization variation of the optical components 215, 225, 230, 265, 270 of an individual channel 220 and calculate from this data a respective pixel arrangement 530, 540 for an individualized defect compensation of the optical element 500.
Furthermore, the optical element 500 can also be equipped with one or more actuators to temporarily induce a strain induced birefringence (not shown in
Furthermore, the optical element 500 may be dynamically inserted in the channel 220 depending on the adjustments of the optical components in the channel 220, in particular as a consequence of a movement of the optical elements 265. This enables a flexible response to various settings of the illumination system 130.
In order to generate a predetermined polarization when superimposing the partial beams 605-620, it may however be required that the partial beam 620 has an elliptically polarization instead of a linear polarization. For this purpose, an optical element 500 is inserted in the channel of the partial beam 620 having one or several arrangements 530, 540 of local persistent modifications which generate a strain induced birefringence with a fixed fast axis and a defined retardation in order to generate in combination with the polarization plate 656 the required polarization of the partial beam 785.
The insertion of the optical element 500 upstream of the polarization plates 650 is beneficial as this arrangement facilitates the orientation of the fast axis of the optical element 500 with respect to the polarization of the partial beam 620. On the other hand, it is also possible to arrange the optical element 500 downstream of the polarization plates 652, 654, 656 when the orientation of its fast axis of the optical element 500 is controlled with respect to the polarization of the partial beam 620.
The impact of the optical element 500 is controlled by the IPS variation of the respective illumination beam 285.
If the two partial beams 235 are superimposed by a respective tilting of the mirrors 240 or 660, the polarization of the superimposed beam is described by the Stokes vector Ssum:
This demonstrates that the retardation introduced by the optical element 500 is also effective when an additional polarization rotation is generated by the polarization plates 650. This means that an optical element 500 inserted in a channel 220 of a partial beam 235 can also compensate polarization defects in various other channels at the superposition of the partial beams 235 even if the other channels have different polarization states due to the action of the polarization plates 652, 654, 656.
It can be useful to prepare a set or polarization plates 652, 654, 656 having a different amount of strain induced birefringence. Furthermore, it is also conceivable to introduce a pixel arrangement 530, 540 in one or more of the optical components 215, 225, 265, 270. In specific situations, a combination of an optical element 500 and a further compensation measure by the introduction of one or more pixel arrangements 530, 540 in one of the optical components 215, 225, 230, 265, 270 can also be used.
Moreover, there may be configurations of the illumination system 130 in which it is more effective to use a plate or a compensation plate acting on more than just one channel 220 instead of the optical element 500. It may be useful to apply a compensation plate for a portion of the channel matrix. For example, a compensation plate may be fabricated for a quarter of the two-dimensional channel matrix for the partial beams 235. Such a compensation plate may for example beneficially be used for a quadrupole setting of the illumination system 130.
In the example of
As already mentioned at the discussion of the optical element 500, if the compensation plate 900 is inserted upstream of the polarization plates 652, 654, 656, the compensation plate 900 needs only to have two orientations of the fast axis which are ±45° relative to the polarization of the partial beams 235, 605-620. The following figures indicate a few conceivable implementation possibilities of a compensation plate 900 in combination with the polarization plates 652, 654, 656.
Saenger, Ingo, Schlesener, Frank
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